Magnetic Resonance Imaging, or MRI, is a well-known imaging technique during which an object, such as a human patient, is placed into an MRI machine and subjected to a uniform magnetic field produced by a polarizing magnet housed within the MRI machine. Radio frequency (RF) pulses, generated by an RF coil housed within the MRI machine, are used to scan target tissue of the patient. MRI signals are radiated by excited nuclei in the target tissue in the intervals between consecutive RF pulses and are sensed by the RF coil. During MRI signal sensing, finely controlled magnetic field gradients are switched rapidly to alter the uniform magnetic field at localized areas thereby to allow spatial localization of MRI signals radiated by selected slices of the target tissue. The sensed MRI signals are in turn digitized and processed to reconstruct images of the target tissue slices using one of many known techniques.
In a system capable of performing MRI, a strong uniform static magnetic field is required in order to align the nuclear spins of the object within a particular imaging volume. This uniform static magnetic field is normally produced by a permanent or coil magnet assembly with a magnetic field strength on the order of 0.1 to 4.7 Tesla within the imaging volume. During sensing, the finely controlled magnetic field gradients imposed in the imaging volume allow for discrimination amongst nuclear spins at different locations. However, inhomogeneities in the static magnetic field within the imaging volume are inseparable from the magnetic field gradients during image acquisition and directly lead to geometric distortions in the resulting images. These distortions are especially detrimental when the MRI system is to be used in conjunction with another procedure that relies on the geometric accuracy of the acquired images, such as, but not limited to, radiation therapy. Consequently, significantly reducing static magnetic field inhomogeneities is extremely important in order to achieve images that are of a high quality and that have a high degree of geometric accuracy. For example, acceptable image quality can be achieved where the level of inhomogeneity is on the order of 10 ppm within the imaging volume.
It is known to reduce axisymmetric and non-axisymmetric static magnetic field inhomogeneities using techniques such as passive shimming. Passive shimming is performed after the magnetic assembly has been manufactured, and involves strategically placing additional pieces of magnetic material in and around the imaging volume. The additional pieces of magnetic material are typically of various shapes including rings, ring segments, cylinders, and prisms. While shimming has in some applications been effective in limiting inhomogeneity in the imaging volume, its effectiveness is limited by the extent to which the initial field inhomogeneities are present after manufacturing. As such, significant constraints are placed on the design of magnet assemblies, and by the requirement of maintaining a suitably large and accessible space within the magnet assembly for the object being examined.
Attempts to circumvent the limitations of passive shimming techniques have been made by improving the design of the manufactured magnet assemblies in order to reduce inherent axisymmetric field inhomogeneities. In the present state of the art for bi-planar magnets, the opposing surfaces of the magnet pole pieces are contoured in such a way that the pole pieces are shaped axisymmetrically about an axis generally extending towards the opposing pole piece surface. For example, the most common such pole piece design is known as a rose-ring design, in which the surface of the pole piece which is closest to the imaging volume is entirely flat with the exception of a ring of magnetic material placed along the periphery of the said pole piece surface. More particularly, a graph of axial distance of the pole piece surface along the axis, versus radial distance from the axis is a line of zero slope with a single vertical step at the radial position of the rose-ring.
One magnet assembly design disclosed in U.S. Pat. No. 5,539,366 consists of axisymmetrically shaped pole pieces for which a graph of axial distance, of the pole piece surface along the axis, versus radial distance from the axis is a piece-wise linear curve, or is a non-linear curve having a continuous slope with at least two sign reversals. Both such designs are limited in that points on the surface regions of the pole pieces located an identical radial distance from the axis are also located a common axial distance along the axis, and thus only axisymmetric magnetic field inhomogeneities can be reduced prior to shimming. Furthermore, these systems are massive and immobile as the pole piece sizes are necessarily very large, and therefore are not suitable for movement relative to the subject being examined.
Often other objects and/or devices are placed in the vicinity of an MRI device. For example, described in PCT Patent Application Publication Number WO 2007/045076 A1 to Fallone et al., the contents of which are incorporated entirely herein by reference, is an integrated external beam radiotherapy and MRI system, wherein a linear accelerator (linac) is coupled to an MRI apparatus for providing simultaneous imaging and treatment. Unfortunately, current state of the art magnet assembly design does not address the effects of including objects or additional therapeutic or diagnostic devices within or proximate to the magnet assemblies, while providing acceptably homogeneous imaging volumes and/or other volumes with specific magnetic field properties, and ensuring that the size of the magnet assembly is manageable. The operation of such additional devices may be affected by the presence and/or characteristics of the magnetic field at their location and may themselves alter the characteristics of the magnetic field in the imaging volume. Furthermore, the incorporation of such objects or devices within or proximate the magnet assembly may require a particular volume of free space to be vacated from the magnet assembly, such as a large hole through the magnet structure, either for placement of the object or device, or for providing a benefit in performance of the object or device itself. For example, it may be advantageous to align the magnetic fields produced by a magnet assembly with the direction of electrons in a linac waveguide (or the protons produced by the linac for proton therapy) for image guided radiotherapy, particularly to reduce subsequent perturbations in patient radiation dosimetry. In general, such vacated volumes significantly affect the magnetic field produced by the magnet assembly and contribute to a highly inhomogeneous field in the imaging volume.
It is therefore an object of the invention to provide a magnet assembly and a method for defining a magnetic field for an imaging volume for that mitigates or obviates at least one of the above-described disadvantages of the prior art.